[PATCH v5 0/2] Add no-hpd property to the cadence bridge

[PATCH v5 0/2] Add no-hpd property to the cadence bridge

[Yashas D]

This series adds 'no-hpd' device tree property support to the Cadence
MHDP8546 bridge driver for boards where the HPD line cannot be used for
hotplug detection.

On TI J721S2 EVMs, the HPD signal is routed to SoC pin AA24
(MCASP1_ACLKX/DP0_HPD). This pin is muxed with the McASP1 audio bit
clock; selecting DP0_HPD breaks audio. There is no alternative pin
carrying the HPD signal which makes it impossible to use HPD
without loosing the audio capabilities.

When 'no-hpd' is set, DRM_BRIDGE_OP_HPD is omitted so the framework
falls back to polling .detect() every ~10 seconds. Monitor presence is
determined via AUX DPCD reads instead of firmware HPD status registers.
The .detect() callback drives cdns_mhdp_update_link_status() on each
poll to keep mhdp->plugged current. At attach time, the driver waits
for firmware to be ready before performing the initial AUX poll since
no interrupt will trigger it.

Changes since v4:
- Corrected the dt-binding Subject

Link to v4: https://lore.kernel.org/all/20260630102610.1849902-1-y-d@ti.com/

Rahul T R (2):
  dt-bindings: display: bridge: cdns,mhdp8546: Add no-hpd property to
    the cadence bridge
  drm: bridge: cdns-mhdp8546: Add no-hpd property

 .../display/bridge/cdns,mhdp8546.yaml         | 17 ++++++
 .../drm/bridge/cadence/cdns-mhdp8546-core.c   | 58 ++++++++++++++++---
 .../drm/bridge/cadence/cdns-mhdp8546-core.h   |  1 +
 3 files changed, 69 insertions(+), 7 deletions(-)

-- 
2.34.1

[PATCH v5 1/2] dt-bindings: display: bridge: cdns,mhdp8546: Add no-hpd property to the cadence bridge

[Yashas D]

From: Rahul T R <r-ravikumar@ti.com>

The mhdp bridge can work without its HPD pin hooked up to the connector,
but the current bridge driver throws an error when hpd line is not
connected to the connector. For such cases, we need an indication for
no-hpd, using which we can bypass the hpd detection and instead use the
auxiliary channels connected to the DP connector to confirm the
connection.
So add no-hpd property to the bindings, to disable hpd when not
connected or cannot be used for hotplug detection.

Signed-off-by: Rahul T R <r-ravikumar@ti.com>
Signed-off-by: Jayesh Choudhary <j-choudhary@ti.com>
Signed-off-by: Yashas D <y-d@ti.com>
---
 .../bindings/display/bridge/cdns,mhdp8546.yaml  | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml b/Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml
index c2b369456e4e..f19a4900ecd8 100644
--- a/Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml
+++ b/Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml
@@ -57,6 +57,23 @@ properties:
   interrupts:
     maxItems: 1
 
+  no-hpd:
+    type: boolean
+    description:
+      Set if the HPD line on the bridge isn't physically connected to the
+      DisplayPort connector or cannot be used for hotplug detection.
+
+      Valid use cases include:
+      - HPD pin not routed to the connector on the PCB
+      - HPD signal muxed with another function (e.g., audio) on the SoC,
+        making it unavailable for hotplug detection
+      - Hardware design where HPD cannot reliably detect monitor presence
+
+      When this property is set, the driver will use auxiliary channel (AUX)
+      DPCD reads to detect monitor connection instead of relying on the
+      hardware HPD signal. Monitor detection will use polling rather than
+      interrupt-driven detection.
+
   ports:
     $ref: /schemas/graph.yaml#/properties/ports
 
-- 
2.34.1

[PATCH v5 2/2] drm: bridge: cdns-mhdp8546: Add no-hpd property

[Yashas D]

From: Rahul T R <r-ravikumar@ti.com>

Add a 'no-hpd' boolean property to support boards where the HPD line
cannot be used for hotplug detection due to hardware limitations.

On TI J721S2 EVMs, the DP0 HPD signal has a MUX conflict with audio
functionality. While the HPD pin is physically connected to GPIO0_18,
routing it to the SoC requires sacrificing audio capability due to pin
muxing constraints. This board-level hardware limitation necessitates
an alternative detection mechanism.

When this property is set, the driver uses auxiliary channel (AUX) DPCD
reads to detect monitor presence instead of hardware HPD signals. The
DRM framework polls the connection status via the .detect() callback,
providing hotplug detection without requiring the HPD pin.

Valid use cases:
- HPD pin not routed to connector on PCB
- HPD signal muxed with another function (e.g., audio) on SoC
- Hardware designs where HPD cannot reliably detect monitor presence

Limitations:
- DP "attention" IRQs from sink will not be received
- Hotplug detection latency increases to ~10 seconds (polling interval)

This follows the same pattern as 'no-hpd' in ti-sn65dsi86 and panel-edp
bindings, providing consistent semantics across the DRM subsystem.

Signed-off-by: Rahul T R <r-ravikumar@ti.com>
Signed-off-by: Jayesh Choudhary <j-choudhary@ti.com>
Signed-off-by: Harikrishna Shenoy <h-shenoy@ti.com>
Signed-off-by: Yashas D <y-d@ti.com>
---
 .../drm/bridge/cadence/cdns-mhdp8546-core.c   | 58 ++++++++++++++++---
 .../drm/bridge/cadence/cdns-mhdp8546-core.h   |  1 +
 2 files changed, 52 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
index 504a3186ebb3..73d5c718c7db 100644
--- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
+++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c
@@ -53,6 +53,8 @@
 #include "cdns-mhdp8546-hdcp.h"
 #include "cdns-mhdp8546-j721e.h"
 
+static int cdns_mhdp_update_link_status(struct cdns_mhdp_device *mhdp);
+
 static void cdns_mhdp_bridge_hpd_enable(struct drm_bridge *bridge)
 {
 	struct cdns_mhdp_device *mhdp = bridge_to_mhdp(bridge);
@@ -698,7 +700,9 @@ static int cdns_mhdp_fw_activate(const struct firmware *fw,
 	 * MHDP_HW_STOPPED happens only due to driver removal when
 	 * bridge should already be detached.
 	 */
-	cdns_mhdp_bridge_hpd_enable(&mhdp->bridge);
+
+	if (!mhdp->no_hpd)
+		cdns_mhdp_bridge_hpd_enable(&mhdp->bridge);
 
 	spin_unlock(&mhdp->start_lock);
 
@@ -788,9 +792,14 @@ static ssize_t cdns_mhdp_transfer(struct drm_dp_aux *aux,
 		ret = cdns_mhdp_dpcd_read(mhdp, msg->address,
 					  msg->buffer, msg->size);
 		if (ret) {
-			dev_dbg(mhdp->dev,
-				"Failed to read DPCD addr %u\n",
-				msg->address);
+			if (mhdp->no_hpd)
+				dev_dbg(mhdp->dev,
+					"Failed to read DPCD addr %u\n",
+					msg->address);
+			else
+				dev_err(mhdp->dev,
+					"Failed to read DPCD addr %u\n",
+					msg->address);
 
 			return ret;
 		}
@@ -1523,6 +1532,26 @@ static int cdns_mhdp_attach(struct drm_bridge *bridge,
 
 	spin_unlock(&mhdp->start_lock);
 
+	if (mhdp->no_hpd) {
+		/*
+		 * In no-hpd mode there is no interrupt to signal firmware
+		 * readiness. The firmware loads asynchronously after probe(),
+		 * so we must wait here until the uCPU is running before
+		 * attempting the first AUX channel poll for monitor presence.
+		 */
+		ret = wait_event_timeout(mhdp->fw_load_wq,
+					 mhdp->hw_state == MHDP_HW_READY,
+					 msecs_to_jiffies(100));
+		if (ret == 0) {
+			dev_err(mhdp->dev, "%s: Timeout waiting for fw loading\n",
+				__func__);
+			return -ETIMEDOUT;
+		}
+
+		cdns_mhdp_update_link_status(mhdp);
+		return 0;
+	}
+
 	/* Enable SW event interrupts */
 	if (hw_ready)
 		cdns_mhdp_bridge_hpd_enable(bridge);
@@ -2013,6 +2042,9 @@ cdns_mhdp_bridge_detect(struct drm_bridge *bridge, struct drm_connector *connect
 {
 	struct cdns_mhdp_device *mhdp = bridge_to_mhdp(bridge);
 
+	if (mhdp->no_hpd)
+		cdns_mhdp_update_link_status(mhdp);
+
 	return cdns_mhdp_detect(mhdp);
 }
 
@@ -2100,7 +2132,16 @@ static int cdns_mhdp_update_link_status(struct cdns_mhdp_device *mhdp)
 
 	mutex_lock(&mhdp->link_mutex);
 
-	mhdp->plugged = cdns_mhdp_detect_hpd(mhdp, &hpd_pulse);
+	if (mhdp->no_hpd) {
+		ret = drm_dp_dpcd_read_link_status(&mhdp->aux, status);
+		hpd_pulse = false;
+		if (ret < 0)
+			mhdp->plugged = false;
+		else
+			mhdp->plugged = true;
+	} else {
+		mhdp->plugged = cdns_mhdp_detect_hpd(mhdp, &hpd_pulse);
+	}
 
 	if (!mhdp->plugged) {
 		cdns_mhdp_link_down(mhdp);
@@ -2288,6 +2329,8 @@ static int cdns_mhdp_probe(struct platform_device *pdev)
 	mhdp->aux.dev = dev;
 	mhdp->aux.transfer = cdns_mhdp_transfer;
 
+	mhdp->no_hpd = of_property_read_bool(dev->of_node, "no-hpd");
+
 	mhdp->regs = devm_platform_ioremap_resource(pdev, 0);
 	if (IS_ERR(mhdp->regs)) {
 		dev_err(dev, "Failed to get memory resource\n");
@@ -2360,8 +2403,9 @@ static int cdns_mhdp_probe(struct platform_device *pdev)
 	mhdp->display_fmt.bpc = 8;
 
 	mhdp->bridge.of_node = pdev->dev.of_node;
-	mhdp->bridge.ops = DRM_BRIDGE_OP_DETECT | DRM_BRIDGE_OP_EDID |
-			   DRM_BRIDGE_OP_HPD;
+	mhdp->bridge.ops = DRM_BRIDGE_OP_DETECT | DRM_BRIDGE_OP_EDID;
+	if (!mhdp->no_hpd)
+		mhdp->bridge.ops |= DRM_BRIDGE_OP_HPD;
 	mhdp->bridge.type = DRM_MODE_CONNECTOR_DisplayPort;
 
 	ret = phy_init(mhdp->phy);
diff --git a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.h b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.h
index b53335b0d22c..24ffb732a207 100644
--- a/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.h
+++ b/drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.h
@@ -388,6 +388,7 @@ struct cdns_mhdp_device {
 
 	bool link_up;
 	bool plugged;
+	bool no_hpd;
 
 	/*
 	 * "start_lock" protects the access to bridge_attached and
-- 
2.34.1

Re: [PATCH v5 1/2] dt-bindings: display: bridge: cdns,mhdp8546: Add no-hpd property to the cadence bridge

[Rob Herring (Arm)]

On Tue, 28 Jul 2026 18:12:23 +0530, Yashas D wrote:
> From: Rahul T R <r-ravikumar@ti.com>
> 
> The mhdp bridge can work without its HPD pin hooked up to the connector,
> but the current bridge driver throws an error when hpd line is not
> connected to the connector. For such cases, we need an indication for
> no-hpd, using which we can bypass the hpd detection and instead use the
> auxiliary channels connected to the DP connector to confirm the
> connection.
> So add no-hpd property to the bindings, to disable hpd when not
> connected or cannot be used for hotplug detection.
> 
> Signed-off-by: Rahul T R <r-ravikumar@ti.com>
> Signed-off-by: Jayesh Choudhary <j-choudhary@ti.com>
> Signed-off-by: Yashas D <y-d@ti.com>
> ---
>  .../bindings/display/bridge/cdns,mhdp8546.yaml  | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 

My bot found errors running 'make dt_binding_check' on your patch:

yamllint warnings/errors:
./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml:66:30: [error] syntax error: mapping values are not allowed here (syntax)

dtschema/dtc warnings/errors:
/builds/robherring/dt-review-ci/linux/Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml: ignoring, error parsing file
./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml: error loading YAML: mapping values are not allowed here
  in "./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml", line 66, column 30
./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml:66:30: mapping values are not allowed here
make[2]: *** Deleting file 'Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.example.dts'
Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml:66:30: mapping values are not allowed here
make[2]: *** [Documentation/devicetree/bindings/Makefile:26: Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.example.dts] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [/builds/robherring/dt-review-ci/linux/Makefile:1703: dt_binding_check] Error 2
make: *** [Makefile:248: __sub-make] Error 2

doc reference errors (make refcheckdocs):

See https://patchwork.kernel.org/project/devicetree/patch/20260728124224.774785-2-y-d@ti.com

The base for the series is generally the latest rc1. A different dependency
should be noted in *this* patch.

If you already ran 'make dt_binding_check' and didn't see the above
error(s), then make sure 'yamllint' is installed and dt-schema is up to
date:

pip3 install dtschema --upgrade

Please check and re-submit after running the above command yourself. Note
that DT_SCHEMA_FILES can be set to your schema file to speed up checking
your schema. However, it must be unset to test all examples with your schema.

Re: Re: [PATCH v5 1/2] dt-bindings: display: bridge: cdns,mhdp8546: Add no-hpd property to the cadence bridge

[Yashas D]

On 28/07/26 19:59, Rob Herring (Arm) wrote:
> On Tue, 28 Jul 2026 18: 12: 23 +0530, Yashas D wrote: > From: Rahul T R 
> <r-ravikumar@ ti. com> > > The mhdp bridge can work without its HPD pin 
> hooked up to the connector, > but the current bridge driver throws an 
> error when hpd
> 
> 
> On Tue, 28 Jul 2026 18:12:23 +0530, Yashas D wrote:
>> From: Rahul T R <r-ravikumar@ti.com>
>> 
>> The mhdp bridge can work without its HPD pin hooked up to the connector,
>> but the current bridge driver throws an error when hpd line is not
>> connected to the connector. For such cases, we need an indication for
>> no-hpd, using which we can bypass the hpd detection and instead use the
>> auxiliary channels connected to the DP connector to confirm the
>> connection.
>> So add no-hpd property to the bindings, to disable hpd when not
>> connected or cannot be used for hotplug detection.
>> 
>> Signed-off-by: Rahul T R <r-ravikumar@ti.com>
>> Signed-off-by: Jayesh Choudhary <j-choudhary@ti.com>
>> Signed-off-by: Yashas D <y-d@ti.com>
>> ---
>>  .../bindings/display/bridge/cdns,mhdp8546.yaml  | 17 +++++++++++++++++
>>  1 file changed, 17 insertions(+)
>> 
> 
> My bot found errors running 'make dt_binding_check' on your patch:
> 
> yamllint warnings/errors:
> ./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml:66:30: [error] syntax error: mapping values are not allowed here (syntax)
> 
> dtschema/dtc warnings/errors:
> /builds/robherring/dt-review-ci/linux/Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml: ignoring, error parsing file
> ./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml: error loading YAML: mapping values are not allowed here
>    in "./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml", line 66, column 30
> ./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml:66:30: mapping values are not allowed here
> make[2]: *** Deleting file 'Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.example.dts'
> Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml:66:30: mapping values are not allowed here
> make[2]: *** [Documentation/devicetree/bindings/Makefile:26: Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.example.dts] Error 1
> make[2]: *** Waiting for unfinished jobs....
> make[1]: *** [/builds/robherring/dt-review-ci/linux/Makefile:1703: dt_binding_check] Error 2
> make: *** [Makefile:248: __sub-make] Error 2
> 
> doc reference errors (make refcheckdocs):
> 
> Seehttps://urldefense.com/v3/__https://patchwork.kernel.org/project/ 
> devicetree/patch/20260728124224.774785-2-y-d@ti.com__;!!G3vK!W- 
> t7VTYjMiEiiyA5HnNwMPXZXIDl6mIiVFQjSxqG20XM1FTyTXYmNfyADL2cjwLH7WdrUA$ <https://urldefense.com/v3/__https://patchwork.kernel.org/project/devicetree/patch/20260728124224.774785-2-y-d@ti.com__;!!G3vK!W-t7VTYjMiEiiyA5HnNwMPXZXIDl6mIiVFQjSxqG20XM1FTyTXYmNfyADL2cjwLH7WdrUA$>
> 
> The base for the series is generally the latest rc1. A different dependency
> should be noted in *this* patch.
> 
> If you already ran 'make dt_binding_check' and didn't see the above
> error(s), then make sure 'yamllint' is installed and dt-schema is up to
> date:
> 
> pip3 install dtschema --upgrade
> 
> Please check and re-submit after running the above command yourself. Note
> that DT_SCHEMA_FILES can be set to your schema file to speed up checking
> your schema. However, it must be unset to test all examples with your schema.
> 

Hi Rob,
My dt_binding_check had passed, will upgrade and run it again and send a 
new patch.

Thanks,
Yashas D

Re: [PATCH v5 1/2] dt-bindings: display: bridge: cdns,mhdp8546: Add no-hpd property to the cadence bridge

[Krzysztof Kozlowski]

On Tue, Jul 28, 2026 at 09:49:47PM +0530, Yashas D wrote:
> 
> 
> On 28/07/26 19:59, Rob Herring (Arm) wrote:
> > On Tue, 28 Jul 2026 18: 12: 23 +0530, Yashas D wrote: > From: Rahul T R
> > <r-ravikumar@ ti. com> > > The mhdp bridge can work without its HPD pin
> > hooked up to the connector, > but the current bridge driver throws an
> > error when hpd
> > 
> > 
> > On Tue, 28 Jul 2026 18:12:23 +0530, Yashas D wrote:
> > > From: Rahul T R <r-ravikumar@ti.com>
> > > 
> > > The mhdp bridge can work without its HPD pin hooked up to the connector,
> > > but the current bridge driver throws an error when hpd line is not
> > > connected to the connector. For such cases, we need an indication for
> > > no-hpd, using which we can bypass the hpd detection and instead use the
> > > auxiliary channels connected to the DP connector to confirm the
> > > connection.
> > > So add no-hpd property to the bindings, to disable hpd when not
> > > connected or cannot be used for hotplug detection.
> > > 
> > > Signed-off-by: Rahul T R <r-ravikumar@ti.com>
> > > Signed-off-by: Jayesh Choudhary <j-choudhary@ti.com>
> > > Signed-off-by: Yashas D <y-d@ti.com>
> > > ---
> > >  .../bindings/display/bridge/cdns,mhdp8546.yaml  | 17 +++++++++++++++++
> > >  1 file changed, 17 insertions(+)
> > > 
> > 
> > My bot found errors running 'make dt_binding_check' on your patch:
> > 
> > yamllint warnings/errors:
> > ./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml:66:30: [error] syntax error: mapping values are not allowed here (syntax)
> > 
> > dtschema/dtc warnings/errors:
> > /builds/robherring/dt-review-ci/linux/Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml: ignoring, error parsing file
> > ./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml: error loading YAML: mapping values are not allowed here
> >    in "./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml", line 66, column 30
> > ./Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml:66:30: mapping values are not allowed here
> > make[2]: *** Deleting file 'Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.example.dts'
> > Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.yaml:66:30: mapping values are not allowed here
> > make[2]: *** [Documentation/devicetree/bindings/Makefile:26: Documentation/devicetree/bindings/display/bridge/cdns,mhdp8546.example.dts] Error 1
> > make[2]: *** Waiting for unfinished jobs....
> > make[1]: *** [/builds/robherring/dt-review-ci/linux/Makefile:1703: dt_binding_check] Error 2
> > make: *** [Makefile:248: __sub-make] Error 2
> > 
> > doc reference errors (make refcheckdocs):
> > 
> > Seehttps://urldefense.com/v3/__https://patchwork.kernel.org/project/
> > devicetree/patch/20260728124224.774785-2-y-d@ti.com__;!!G3vK!W-
> > t7VTYjMiEiiyA5HnNwMPXZXIDl6mIiVFQjSxqG20XM1FTyTXYmNfyADL2cjwLH7WdrUA$ <https://urldefense.com/v3/__https://patchwork.kernel.org/project/devicetree/patch/20260728124224.774785-2-y-d@ti.com__;!!G3vK!W-t7VTYjMiEiiyA5HnNwMPXZXIDl6mIiVFQjSxqG20XM1FTyTXYmNfyADL2cjwLH7WdrUA$>
> > 
> > The base for the series is generally the latest rc1. A different dependency
> > should be noted in *this* patch.
> > 
> > If you already ran 'make dt_binding_check' and didn't see the above
> > error(s), then make sure 'yamllint' is installed and dt-schema is up to
> > date:
> > 
> > pip3 install dtschema --upgrade
> > 
> > Please check and re-submit after running the above command yourself. Note
> > that DT_SCHEMA_FILES can be set to your schema file to speed up checking
> > your schema. However, it must be unset to test all examples with your schema.
> > 
> 
> Hi Rob,
> My dt_binding_check had passed, will upgrade and run it again and send a new
> patch.

I doubt, code has obvious syntax issues...

Best regards,
Krzysztof